1. Field of the Invention
The invention relates to a process for production of sorbitol from hydrolyzed starch solution by a two-step hydrogenation employing Raney nickel catalyst under neutral to mildly acidic conditions followed by use of a ruthenium catalyst under more acidic conditions.
2. Description of the Prior Art
Processes for production of sorbitol by hydrogenation of glucose in aqueous solution are well known in the art. For example, U.S. Pat. Nos. 3,538,019, 3,670,035 and Faith, Keyes and Clark, "Industrial Chemicals," Lowenheim et al., editors, Wiley-Interscience, New York, 4th edition, 1975, pp. 774-778, disclose the use of Raney nickel and supported nickel catalysts for commercial production of sorbitol from glucose syrups.
Ruthenium on various support materials is also known to be an effective catalyst for conversion of glucose to sorbitol from the disclosures of U.S. Pat. Nos. 2,868,847; 3,963,788; 3,963,789; 4,380,679 and W. German Application No. 3,144,320.
In a recent review by Kieboom and van Bekkum in "Starch Conversion Technology," van Beynum and Roels, editors, M. Dekker, Inc., New York 1985, page 278, typical conditions for a batch process for the manufacture of sorbitol by hydrogenation of glucose using a nickel catalyst are given as: 45-50% (w/v) aqueous solution of glucose, 120.degree.-150.degree. C., 30-70 atmospheres hydrogen, pH 5-6, with 3 to 6% Raney nickel based on glucose. In continuous processes higher hydrogen pressures such as 170 atmospheres, with a supported nickel catalyst are typical.
While nickel catalysts are relatively inexpensive, they suffer from the disadvantages of requiring large amounts of catalyst which can not be readily recycled to the next batch without reprocessing, and the fact that the hydrogenation is best carried out at a nearly neutral pH. Under these neutral pH conditions the residual oligosaccharides present in inexpensive glucose syrups (such as those obtained by starch hydrolysis) are not hydrolyzed to glucose, but are hydrogenated to give high levels of reduced oligosaccharides in the product.
Ruthenium catalysts are more expensive than nickel catalysts but are known to require considerably less on a weight basis for each batch. They have been known to be recycled for a few runs, but then the catalyst must be regenerated or the ruthenium metal recovered and fresh catalyst prepared.
While hydrogenation of glucose to form sorbitol in solutions at pH 2.5 to 4.5 are known, a lower pH is required for reasonably fast hydrolysis of oligosaccharides in starch hydrolyzate syrups to glucose or for hydrolysis of reduced oligosaccharides during the hydrogenation. Prior attempts to use ruthenium catalysts at a pH below 2.5 have resulted in loss of activity of the catalyst after several runs.
The term "polysaccharide" as used herein includes those saccharides containing more than one monosaccharide unit. This term also incompasses the subclass of oligosaccharides which contain, for the purposes of this specification, from two to about ten monosaccharide units. Examples of oligosaccharides are the disaccharides maltose, lactose, cellobiose and sucrose; and typical trisaccharides are raffinos and maltotriose. Hydrolysis of a disaccharide such as maltose provides two molecules of glucose per molecule of maltose. Likewise, complete hydrolysis of each molecule of the trisaccharide maltotriose provides three molecules of glucose and maltotetraose provides four glucose molecules.
Hydrogenation of an oligosaccharide such as maltose, maltotriose, maltotetraose or high oligomers of glucose (which are ordinarily present at levles from about 2-5 percent in commercially available, low cost, starch hydrolyzates such as those provided in U.S. Pat. No. 4,107,363) lead to a reduced oligosaccharide in which only the terminal aldehyde or hemiacetal groups are reduced. Thus, maltose is reduced to maltitol or isomaltitol maltotriose to a diglucosyl-sorbitol (maltriol) and the linear glucose tetramer is reduced to the corresponding triglucosylsorbitol. Hydrolysis of a reduced diaccharide molecule such as maltitol gives rise to one molecule of glucose and one molecule of sorbitol, while hydrolysis of the reduced trisaccharide, maltrotriol, affords two molecules of glucose and one of sorbitol for each molecule of maltotriol.
In the prior art methods for production of sorbitol substantial levels of reduced oligosaccharides, approximately equivalent to the level of oligosaccharide present in the starch hydrolyzate employed as starting material, are obtained.
Ordinarily, the starting material for commercial manufacture of sorbitol is either pure glucose or a less expensive starch hydrolyzate. Typical such starch hydrolyzates are those disclosed in U.S. Pat. No. 4,017,363 which provide glucose syrups of 98% or higher purity.
The glucose or dextrose content of glucose solutions or syrups can be expressed in terms of the % dextrose or as the Dextrose Equivalent (DE). The latter term is more commonly employed with starch hydrolyzates and is preferred herein.
The DE of a substance is defined as ##EQU1##
Low cost starch hydrolyzates obtained from corn or wheat starch are readily available in commerce. The preferred starch hydrolyzates for production of sorbitol by prior art methods are corn starch hydrolyzates having a DE of 95 or higher.